Farm machinery such as a plow is mounted to the rear of the body of an agricultural tractor via a link mechanism that is hitched at three points. The link mechanism is driven by a hydraulic actuator which is usually a single-acting cylinder. This hydraulic actuator is controlled by a hydraulic control circuit including hydraulic pumps and control valves, as described in Japanese Patent Laid-Open No. 28,501/1987.
In recent years, even agricultural tractors having relatively small horsepower, e.g., of the order of 30 H.P., have been required to do not only normal plow works but also other associated works. Thus, many tractors are equipped with one or more hydraulically operated attachments independent of the front of the vehicle and of the body, as well as the aforementioned hydraulic actuator that acts as a main hydraulic actuator.
One typical example of attachment mounted on the body of a tractor is a front loader. A typical example of a farm attachment independent of the body of a tractor is a water pump. A hydraulic actuator (hereinafter referred to as an auxiliary hydraulic actuator) for the former attachment comprises a plurality of hydraulic cylinders. A hydraulic actuator for the latter attachment is a hydraulic motor. Of course, the auxiliary hydraulic actuator is controlled by a control valve. Generally, therefore, the whole hydraulic control circuit comprises a control valve for the main hydraulic actuator and a second control valve for the auxiliary hydraulic actuator. The control valves are connected in series with a hydraulic pump.
For an agricultural tractor, the size of the vehicle, or horsepower, is first determined. The specifications of the farm machinery are then determined according to the size. Usually, the flow rate of the hydraulic pump, the capacity of the hydraulic cylinder, and the capacities of the control valves are determined to set the rising speed and the lowering speed of the farm machinery within a given range. Accordingly, if the flow rate needed to control the auxiliary hydraulic actuator that drives the farm attachment lies within the range of the flow rate of the hydraulic pump, then no problems will occur.
Sometimes, however, a flow rate larger than the flow rate of the hydraulic pump is needed in controlling the auxiliary hydraulic actuator, i.e., the flow rate is increased. In reality, a hydraulic cylinder of a large capacity typified by a hydraulic cylinder for a front loader is employed as an auxiliary hydraulic actuator. One conceivable method of satisfying the above requirement is to use a hydraulic pump of a large capacity, for providing large flow rates. In this case, however, excessive flow rate is supplied to the main hydraulic actuator when this actuator is operated without operating the auxiliary hydraulic actuator. As a result, the speed of the farm machinery becomes too high, and the machinery is controlled with decreased accuracy.
Japanese Utility Model Laid-Open No. 56,802/1987 discloses a control circuit in which delivery passages extending from two hydraulic pumps are connected with a hydraulic actuator via a plurality of multistage combining selector valves to control the speed of a single hydraulic actuator. This control circuit is unable to control the speeds of two or more hydraulic devices simultaneously. More importantly, it is impossible for the circuit to operate the main hydraulic actuator without changing the predetermined velocity and to smoothly operate the auxiliary hydraulic actuator which needs a large flow rate only when the need arises.